: The causative microbial pathogens and the fundamental host responses in teeth with pulp necrosis and periapical (PA) lesions have not been adequately characterized. Much less is known about these host/pathogen interactions in patients with type 1 diabetes mellitus (DM). We hypothesize that patients with type 1 DM have more symptomatic and/or therapy-resistant PA lesions, which may or may not be related to the degree of their glycemic control. We also hypothesize that in root canals with necrotic pulp of these patients, more virulent and more numerous species of endodontopathic microorganisms are present. The aims of this research project are: (1) Determine the effects of type 1 DM on the development of symptoms in patients with pulp necrosis and apical periodontitis, and on the resolution of the periapical lesion after one year, and (2) Determine the effects of type 1 DM on the prevalence of pathogenic bacteria and Candida albicans before and after root canal preparation in these teeth, using sensitive molecular techniques. Endodontic patients who have type 1 Dm or are non-diabetic, and who have at least one tooth with pulp necrosis and a periapical lesion, will be recruited for this study. Documentation of peri-operative symptoms will be done using visual analog scale measure s for pain and swelling. Endodontic treatment will be completed in a standardized manner. Microbial samples form root canals of the teeth treated will be subjected to PCR amplification of the 16SrRNA gene of selected pathogenic bacteria or 18SrRna gene Candida. Analysis will include universal eubacterial identification, followed by species-level identification of the selected pathogenic organisms using specific oligonucleotide PCR primers. Molecular sequencing will be performed on the PCR product generated with universal bacterial primers, in order to identify other root canal bacterial species present. Sampling and molecular identification will be repeated just before oburation of the root canals. Patients will have follow-up examinations one year post-operatively, with standardized periapical radiographs. Type 1 diabetic and non-diabetic patients will be compared as to peri-operative symptoms, treatment outcome, number of microbial species in the root canals preoperatively and following canal instrumentation, and the association of microbial species and their quantitative measures with symptoms and treatment outcome. Microbial and clinical factors will also be related to the degree of glycemic control of the diabetic patients.